本期是光合作用的第三期：光與光合色素。只想看生物內(nèi)容的朋友可以跳過本期。

如果有不太明白的或者有錯誤的地方隨時來找UP主喔~ 文集本部分的參考文獻 Essential Cell Biology, 5th ed. Alberts, et al.?2019. 部分內(nèi)容來自 khanacademy 與維基百科。本章的內(nèi)容很大程度上參考了 khanacademy.

14c Light and Photosynthetic Pigments?

The human body can't make much use of solar energy, aside from producing a little Vitamin D (a vitamin synthesized in the skin in the presence of sunlight).

Plants, on the other hand, are experts at capturing light energy and using it to make sugars through photosynthesis. This process begins with the absorption of light by specialized organic molecules, called pigments, that are found in the chloroplasts of plant cells.

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Light Energy

Light is a form of electromagnetic radiation, a type of energy that travels in?waves. Other kinds of electromagnetic radiation that we encounter in our daily lives include radio waves, microwaves, and X-rays. Together, all the types of electromagnetic radiation make up the electromagnetic spectrum.

Every electromagnetic wave has a particular wavelength, or distance from one crest to the next, and different types of radiation have different characteristic ranges of wavelengths (as shown in the diagram below). Types of radiation with long wavelengths, such as radio waves, carry less energy than types of radiation with short wavelengths, such as X-rays.

The visible?spectrum?is the only part of the electromagnetic spectrum that can be seen by the human eye. It includes electromagnetic radiation whose wavelength is betWeen about 400 nm and 700 nm. Visible light from the sun appears white, but it’s actually made up of multiple wavelengths (colors) of light.

You can see these different colors when white light passes through a prism: because the different wavelengths of light are bent at different angles as they pass through the?prism, they spread out and form what we see as a rainbow. Red light has the longest wavelength and the least energy, while violet light has the shortest wavelength and the most energy.

Although light and other forms of electromagnetic radiation act as waves under many conditions, they can behave as particles under others. Each particle of electromagnetic radiation, called a photon, has certain amount of energy. Types of radiation with short wavelengths have high-energy photons, whereas types of radiation with long wavelengths have low-energy photons.

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Pigments

In photosynthesis, the sun’s energy is converted to chemical energy by photosynthetic organisms. However, the various wavelengths in sunlight are not all used equally in photosynthesis. Instead, photosynthetic organisms contain light-absorbing molecules called pigments that absorb only specific wavelengths of visible light, while reflecting others.

The set of wavelengths absorbed by a pigment is its absorption spectrum. In the?diagram above,?you can see the?absorption spectra?of three key pigments in photosynthesis: chlorophyll?a, chlorophyll?b, and β-carotene. The set of wavelengths that a pigment doesn't absorb are reflected, and the reflected light is what we see as color. For instance, plants appear green to us because they contain many chlorophyll?a?and?b?molecules, which reflect green light.

Most photosynthetic organisms have a variety of different pigments, so they can absorb energy from a wide range of wavelengths. Here, we'll look at tWo groups of pigments that are important in plants:?chlorophylls and carotenoids.

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Chlorophylls

There are five main types of?chlorophylls: chlorophylls?a,?b,?c?and?d, plus a related molecule found in prokaryotes called bacteriochlorophyll. In plants, chlorophyll?a?and chlorophyll?b?are the main photosynthetic pigments. Chlorophyll molecules absorb blue and red wavelengths, as shown by the peaks in the absorption spectra?above.

Structurally, chlorophyll molecules include a hydrophobic ("water-fearing") tail that inserts into the thylakoid membrane and a porphyrin ring head (a circular group of atoms surrounding a magnesium ion) that absorbs light.

Although both chlorophyll?a?and chlorophyll?b?absorb light, chlorophyll?a?plays a unique and crucial role in converting light energy to chemical energy?in the light reactions during photosynthesis. All photosynthetic plants, algae, and cyanobacteria contain chlorophyll?a, whereas only plants and green algae contain chlorophyll?b, along with a few types of cyanobacteria.

Because of the central role of chlorophyll?a?in photosynthesis, all pigments used in addition to chlorophyll?a?are known as accessory pigments?---?including other chlorophylls, as well as other classes of pigments like the carotenoids. The use of accessory pigments allows a broader range of wavelengths to be absorbed, and thus, more energy to be captured from sunlight.

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Carotenoids

Carotenoids are another key group of pigments that absorb violet and blue-green light (see spectrum graph above). The brightly colored carotenoids found in fruit?---?such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)?---?are often used as advertisements to attract animals, which can help disperse the plant's seeds.

In photosynthesis, carotenoids help?to?capture light, but they also have an important role in getting rid of excess light energy. When a leaf is exposed to full sun, it receives a huge amount of energy; if that energy is not handled properly, it can damage the photosynthetic machinery. Carotenoids in chloroplasts help absorb the excess energy and dissipate it as heat.

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Pigment absorbs Light

When a pigment absorbs a photon of light, it becomes excited, meaning that it has extra energy and is no longer in its normal, or ground, state. At a subatomic level, excitation is when an electron is bumped into a higher-energy orbital that lies further from the nucleus.

Only a photon with just the right amount of energy to bump an electron betWeen orbitals can excite a pigment. In fact, this is why different pigments absorb different wavelengths of light: the "energy gaps" betWeen the orbitals are different in each pigment, meaning that photons of different wavelengths are needed in each case to provide an energy boost that matches the gap.

本次內(nèi)容到此結(jié)束，感謝閱讀！下一期內(nèi)容將是光合作用的第四節(jié)：光反應(yīng)。

作者：離久-張所長